Microphone biasing circuitry and method thereof

ABSTRACT

A host device for use with a removable peripheral apparatus having a microphone, and to the biasing circuitry for said microphone. The host device may have a device connector for forming a mating connection with a respective peripheral connector. A source of bias is arranged to supply an electrical bias to a device microphone contact of the device connector via a biasing path. A capacitor is connected between a reference voltage node and a capacitor node of the biasing path. A first switch is located between the capacitor node and the device microphone contact. Detection circuitry detects disconnection of the peripheral connector and device connector; and control circuitry controls the switch to disable the biasing path.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to microphone biasing circuitry and methods forreducing or removing unwanted bias on a microphone contact of aconnector (for connecting to an external microphone) as a removableperipheral apparatus is removed.

2. Description of the Related Art

Many contemporary electronic devices have the facility to connect withperipheral audio devices. For instance, mobile telephones, tablets,laptop computers and the like are examples of electronic devices thatare operable with peripheral audio devices such as a headset, forexample, that is external to and distinct from the electronic device.Headsets typically comprise mono or stereo speakers for audio playbackand a microphone for voice communication.

Such external peripheral audio devices are often connected via a matingconnector such as a plug and socket arrangement. For instance, manyheadsets have a 3.5 mm jack plug for connection to a suitable jacksocket on the host electronic device. A well-known arrangement for ajack plug and its associated socket is TRRS (Tip-Ring-Ring-Sleeve),which has four contacts for left audio, right audio, microphone, andground. In one known arrangement, the tip (T) and first ring (R1) areused for left (L) and right (R) audio, for example left and rightloudspeakers, with the second ring (R2) and sleeve (S) used for themicrophone (M) and ground (G) respectively. It will be appreciated thatdifferent arrangements for the left and right audio, microphone, andground contacts are also possible.

In use a microphone bias is supplied by the host device to themicrophone (M) contact of the socket so that when a plug (which isconnected to a headset with an external microphone) is plugged into thecompatible socket the external microphone is suitably biased and isready for use.

Using the well-known arrangement described above as an example forillustrating the problem, as the plug is removed from a socket some ofthe contacts (T, R1, R2, S) on the plug will be moved past and makecontact with other various and different contacts (T, R1, R2, S) of thesocket. This can lead to undesired effects if there is still a voltagepresent on a socket contact. For example, when the plug is removed fromthe socket the tip contact T and first ring contact R1 of the plug,which are connected to the speakers (L, R) of the peripheral apparatus,will make contact with the socket contact R2 contact used to providebias for the external microphone. If there is still a bias present atthis socket contact R2 then this unwanted bias causes the speakers tooperate in an undesirable fashion, e.g. to output an audible artefactsuch as a pop or click sound.

FIGS. 1 a and 1 b illustrate this effect. FIG. 1 a shows a jack plug 110fully inserted into a jack socket. The jack socket comprises contactpins 113, 115, 117, 119 serving as contacts T, R1, R2 and S of thesocket respectively. (The remainder of the socket mechanical structureis omitted for clarity). The jack plug has four contact areas, 112, 114,116 and 118 serving as contacts T, R1, R2, S of the plug respectively.In this example these plug contacts may be connected to a first (e.g.left) speaker L, a second (e.g. right) speaker R, microphone M, andcommon ground return path G respectively as shown in FIG. 1 c whichillustrates the connections in the removable peripheral apparatus.However the configuration of the socket contacts and the plug contactsmay differ.

FIG. 1 a shows the jack plug as being fully inserted into the socketwith each plug contact making contact with the correspondingly correctsocket contact (and only that socket contact). Circuitry may beconnected to the four contacts of the socket consistent with thisconnection, for instance microphone bias and amplifier circuitryconnected to the socket R2 contact, a ground connection to the socket Scontact, and right- and left-channel speaker driver amplifiers to thesocket R1 and T contacts.

The socket may also comprise some circuitry, for instance anormally-open switch 111 which is closed when the jack plug is fullyinserted, to allow generation of a jack-insert detection signal, forinstance by means of an external pull-up resistor connected to one poleof the switch, to indicate whether there is a plug fully inserted intothe socket or not.

FIG. 1 b shows the plug having been partially removed. In this state theplug sleeve (plug S contact 118) connected to the common ground return Ghas been completely disconnected from the socket. The plug R2 contact116, connected to the microphone, is now in contact with the socket Scontact 119, which is grounded. The plug R1 contact 114, connected tothe second speaker (R), is now in contact with the socket R2 contact117, itself connected to the microphone bias circuitry. Therefore therewill be a bias on the socket R2 contact 117, and such bias will causecurrent to flow from this contact, through the second speaker R andmicrophone M to ground via socket S contact 119.

Furthermore when the jack plug is pulled out further the plug T contact112 connected to first speaker L will (at least briefly) make contactwith socket R2 contact 117 connected to the microphone bias, and theplug R1 contact 114 connected to second speaker R will make contact withsocket R2 contact 117 connected to the microphone bias. Thus it can beseen that removing the plug will result in the microphone bias beingtemporarily applied to both the left (L) and right (R) audio speakercontacts (T, S1) on the plug which will result in current passingthrough both speakers via their common connection, which may result inan unwanted sound, i.e. an unwanted audio artefact.

In both FIGS. 1 a and 1 b is illustrated a typical configuration ofmicrophone biasing circuitry. In this example microphone biasingcircuitry 102 such as a buffer amplifier outputs, via resistor 121, abias for the external microphone. The microphone output, typically aJFET source follower with gate coupled to a capacitive electret (orMEMS) acoustic transducer, superimposes an AC signal on the d.c. biasprovided. The input circuitry 103 buffers and amplifies the AC signalproduced by the microphone in use. The DC blocking capacitor 124 allowsthe audio signal from the microphone to pass though to the inputamplifier, but blocks the quiescent d.c. level of the microphone outputfrom the d.c. bias of the input amplifier to allow the latter to be setindependently at a convenient level, typically half the supply voltage.

Shown in FIG. 2 is another known configuration for biasing a microphoneof a removable peripheral apparatus. For simplicity, the connectorarrangement, e.g. plug and socket is omitted. The biasing circuitrywill, in use, be connected to a suitable contact on a device connector,such as a microphone socket contact, i.e. a socket contact to which amicrophone is anticipated to be connected via a corresponding microphonejack plug contact. These contacts may correspond to the R2 contacts 117and 116 in the example of FIGS. 1 a and 1 b, but as noted could be othercontacts in other examples.

The microphone biasing circuitry 102, generating the bias for theexternal microphone, connects to the microphone (M) socket contact 117(R2) via two series connected resistors 222 and 223. Between the tworesistors 222, 223 there is a capacitor node to which the first plate ofa capacitor 225 connects. The other plate of the capacitor 225 connectsto a reference voltage node, in this case a ground node. This capacitor225 serves as a low pass filter to remove any noise which could bepicked up on the microphone bias relative to this ground node and wouldotherwise appear as a spurious component of the input signal. In mobiletelephones for example, a significant source of noise is due to the GSMRF power amplifiers, which creates TDMA noise (time division multipleaccess noise) due to supply current modulation appearing on the groundreturn paths for example at a frequency of 217 Hz. In order to preventthis noise being picked up by the microphone circuitry the capacitor 225has to be large enough to significantly attenuate signals of a frequencyof 217 Hz. This means that during operation the capacitor will store anappropriate amount of charge which will eventually have to be dischargedat some point in time.

In order to at least reduce the unwanted bias at the microphone socketcontact the microphone biasing circuitry 102 may be disabled, forexample by removing supply power to the amplifier, as the jack plug isremoved from the socket. However, the large capacitor 225 will take timeto discharge, and meanwhile the charge present on the capacitor 225 willstill create an unwanted bias at the microphone socket contact. Thisunwanted bias from capacitor 225 will then operate the first and secondspeakers connected to the jack plug in an undesired fashion as describedabove.

SUMMARY OF THE INVENTION

Embodiments of the present invention are aimed at reducing the bias atthe microphone contact of the connector of the host device in responseto a removable peripheral apparatus being removed from a socket.

Thus according to the present invention there is provided a host devicefor use with a removable peripheral apparatus having a microphone, thehost device comprising: a device connector capable of forming a matingconnection with a respective peripheral connector of the removableperipheral device, the device connector having a device microphonecontact; a biasing path for supplying an electrical bias to said devicemicrophone contact; a capacitor connected between a reference voltagenode and a capacitor node of said biasing path; a first switch locatedin the biasing path between the capacitor node and the device microphonecontact, said switch being operable to disable the biasing path;detection circuitry for detecting disconnection of the peripheralconnector and device connector; and control circuitry for controllingsaid first switch to disable the biasing path in response to detectionof disconnection of the peripheral connector and device connector.

The device may further comprise biasing circuitry operable to supplysaid electrical bias to the biasing path via a bias node.

Preferably said control circuitry is configured to operate said firstswitch to disable the biasing path within 100 μs of said detection ofdisconnection.

In some embodiments the first switch may be closed only when amicrophone contact of the peripheral connector is detected to beconnected to said device microphone contact.

The device may further comprise a second switch operable to connect thedevice microphone contact to a second reference voltage, which maysubstantially the same voltage as the reference voltage node. The secondswitch may be operable to connect the device microphone contact to thereference voltage node. In use, the voltage at the reference voltagenode may be a ground voltage. The second switch may be controlled bysaid control circuitry, and operates to connect said device microphonecontact to ground in response to the detection of disconnection.

When biasing circuitry is present the control circuitry may beconfigured to disable the biasing circuitry in response to saiddetection of disconnection.

There may additionally be input circuitry for receiving an input signalfrom said device microphone contact. The control circuitry may beconfigured to disable the input circuitry in response to said detectionof disconnection. A DC blocking capacitor may be situated between thedevice microphone contact and the input circuitry. In some embodimentsthere may an input switch located between the DC blocking capacitor andthe input circuitry, which operates in response to said detection ofdisconnection. The input switch may be located between a resistor andthe input circuitry. There may additionally be electrostatic dischargeprotection circuitry connected to an ESD node located between the DCblocking capacitor and the input circuitry, said electrostatic dischargeprotection circuitry comprising a plurality of diodes connected inseries.

The biasing circuitry, second switch and input circuitry may, togetherwith the first switch, form at least part of an integrated circuit.

In some embodiments there may be one or more resistors connected in saidbiasing path, such as two or more two or more resistors connected inseries. The capacitor node may be located between two resistorsconnected in series. The first switch may be located between two of saidtwo or more resistors connected in series or may be located between theresistors and the device microphone contact.

In some embodiments there may be a third switch in parallel with thecapacitor for connecting said capacitor node to said reference voltagenode. The third switch may be controlled by the control circuitry, andoperated to connect the capacitor node to the reference voltage node inresponse to said detection of disconnection.

The biasing circuitry may generate bias at the bias node with respect tothe voltage at a ground contact of said device connector. The referencevoltage node may be connected to said ground contact of said deviceconnector.

The device connector may comprise a socket for receiving a jack plugperipheral connector and the detection circuitry may comprise a jackplug detection circuitry for detecting when a jack plug is fullyconnected into said socket. The detection circuitry may be configured tomonitor the electrical properties of at least one contact of the deviceconnector to detect said disconnection, for example the electricalproperties of the device microphone contact. In some embodiments thedetection circuitry may comprise a mechanical detection switch fordetecting whether the removable peripheral apparatus is fully connected.

The host device may further comprise: an alternative bias path forsupplying an electrical bias to an alternative device contact of saiddevice connector; and a microphone detect circuit for detecting, onconnection of the device connector and the peripheral connector whetheran external microphone is connected to said device microphone contact orsaid alternative device contact; wherein the host device is operable sothat, in use, said biasing path or said alternative bias path can beselectively enabled. The biasing path may comprise a first path sectionand a second path section wherein said first path section is shared withthe alternative bias path and the second path section is not shared withthe alternative bias path wherein said first switch is located in thesecond path section and is operated to disable the biasing path when thealternative bias path is selectively enabled. When the alternative biaspath is selectively enabled the device microphone contact may beconnected to ground. The alternative bias path may comprise a fourthswitch for disabling the alternative bias path and the control circuitrymay be configured to control the fourth switch to disable thealternative bias path in response to detection of disconnection of theperipheral connector and device connector.

The device connector may be a socket for receiving a jack plugperipheral connector, such as a 3.5 mm jack plug peripheral connector.The device connector may be configured to receive a peripheral connectorhaving at least three poles arranged in a tip-ring-sleeve (TRS)arrangement or at least four poles arranged in a tip-ring-ring-sleeve(TRRS) arrangement.

The host device may be at least one of: a portable computing device; alaptop computer; a personal data assistant; a personal media player; anmp3 player; a portable television; a mobile communications device; amobile telephone; a navigation aid; a GPS device; a game console; atablet computer. The host device may be operable with a removableperipheral apparatus which comprises a headset.

In another aspect of the invention there is provided a method ofoperating a host device connected to a removable peripheral apparatushaving a microphone via a device connector forming a mating connectionwith a respective peripheral connector of the removable peripheraldevice, the method comprising: supplying bias to a device microphonecontact of the device connector via a biasing path, wherein a capacitoris connected between a capacitor node of the biasing path and areference voltage; detecting disconnection of the peripheral connectorand the device connector; and operating a first switch to disable thebiasing path in response to said detection of disconnections; whereinsaid first switch is located in the biasing path between the capacitornode and the device microphone contact.

The method may be implemented in any of the embodiments described abovewith reference to the first aspect of the invention. In particular themethod may further comprise: operating a second switch to connect saiddevice microphone contact to the reference voltage node, in response tosaid detection of disconnection.

Embodiments of the present invention also relate to the biasingarrangement itself. Thus in another aspect of the invention there isprovided a circuit for biasing a microphone, comprising: a microphoneconnector node for, in use, connecting to a microphone contact, abiasing path for, in use, supplying an electric bias to said microphoneconnector node, a first switch located in the biasing path; controlcircuitry, wherein said control circuitry is configured to control thefirst switch in use to disconnect the microphone connector node from thebiasing path in response to a detection that a microphone contact isdisconnected from said microphone connector node.

As mentioned the first switch may form part of an integrated circuitwith the biasing circuitry. Thus in a further aspect of the inventionthere is provided an integrated circuit comprising: microphone biasingcircuitry for outputting a biasing signal at a first terminal; a secondterminal, for, in use, receiving said biasing signal via an external RCnetwork, a third terminal, for, in use, outputting said biasing signalto a external microphone contact, a switch operable to connect saidsecond terminal to said third terminal, switch control circuitry foroperating said switch to disconnect said second and third terminals inresponse to a detection that a microphone contact is electricallydisconnected from said third terminal.

In general embodiments of the invention provide a bias reducing circuit,comprising, a connector for connecting an external microphone, saidconnector having a device microphone contact; and switching circuitryfor isolating the device microphone contact from any source of bias,wherein said switching circuitry operates in response to said externalmicrophone being electrically disconnected from said microphone contact.

The bias reducing circuit may further comprising detection circuitry fordetecting whether an external microphone is electrically connected tosaid microphone contact.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described by way of example only with respectto the following drawings, of which:

FIG. 1 a and 1 b illustrates a typical biasing circuitry and jack plugarrangement, with different positions of the jack plug illustrated;

FIG. 1 c illustrates one example of the plug connections in a removableperipheral apparatus;

FIG. 2 illustrates another known biasing arrangement;

FIG. 3 illustrates a circuit for biasing a microphone according to anembodiment the present invention;

FIG. 4 illustrates a circuit for biasing a microphone according toanother embodiment the present invention;

FIG. 5 a illustrates a further embodiment of the invention including aswitch for connecting a microphone socket contact to ground;

FIG. 5 b illustrates a one arrangement for microphone input circuitry;

FIG. 5 c illustrates an embodiment of microphone input circuitryaccording to an embodiment of the present invention;

FIG. 5 d illustrates an alternative embodiment of microphone inputcircuitry;

FIG. 6 a illustrates an embodiment where the microphone biasingcircuitry and input circuitry may be disabled;

FIG. 6 b illustrates another embodiment of the present inventionhighlighting the connections to ground;

FIG. 7 illustrates an embodiment operable with jack plug havingdifferent configurations;

FIG. 8 illustrates an embodiment of the invention implemented as anintegrated circuit; and

FIG. 9 illustrates a host device according to an embodiment of theinvention connecting to a removable peripheral apparatus.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As mentioned above, a host device for use with a removable peripheralapparatus may have a device connector capable of forming a matingconnection with a respective peripheral connector of the removableperipheral apparatus, for instance a plug and socket connectorarrangement, and when the device connector and peripheral connector arefully connected a device microphone contact (of the device connector)may be electrically connected to a corresponding microphone contact ofthe peripheral connector. An electrical bias is supplied to the devicemicrophone contact of such a device connector and, upon separation ofthe connectors, the microphone bias may be applied to the speakers ofthe peripheral apparatus during disconnection, leading to undesiredaudio effects.

In embodiments of the present invention a switch is used to disable thebiasing path to the device microphone contact upon detection that theperipheral connector is in the process of being disconnected from thedevice connector, e.g. the plug is being removed from the socket. Inembodiments of the invention a biasing path is arranged to supply anelectrical bias, by means of a current source or voltage source, to thedevice microphone contact, i.e. the device contact to which a peripheralmicrophone contact of a removable peripheral apparatus may beelectrically connected in use, and a first switch is located in thisbiasing path. A capacitor is coupled between a capacitor node (which islocated in the biasing path) and a reference voltage. The first switchis located between the capacitor node and the device microphone contactand control circuitry operates the first switch to disable the biasingpath in response to detection of disconnection of the peripheralconnector and device connector. The circuitry may therefore comprisemeans for detecting disconnection of the peripheral connector and deviceconnector, such as a jack detect or the like.

The first switch therefore operates to disconnect, i.e. isolate, thedevice microphone (M) contact (R2) from the source of bias and thecharged capacitor (coupled to the capacitor node), thus removing thebias created by those two components from the device microphone contact(R2). Upon the detection of disconnection of the peripheral connectorand device connector by any suitable detection circuitry, the firstswitch is operated, so that the biasing components are disconnected,i.e. isolated, before any other contacts of the peripheral connector ofthe removable peripheral apparatus make electrical connection with thedevice microphone contact. In other words the detection circuitry (themeans for detecting disconnection of the device and peripheralconnectors) detects the onset of disconnection, e.g. partial separationof the device and peripheral connectors, and the first switch isoperated at this stage to disable the biasing path before the othercontacts of the peripheral connector make any contact with the devicemicrophone contact. Therefore, no unwanted bias from the biasing pathwill affect the left and right speakers, for example, when jack plugcontacts (T, R1) pass over a microphone socket contact (R2), asdiscussed above in relation to FIGS. 1 a, 1 b, and 2.

Embodiments of the present invention will be described in relation to ajack plug connector of a removable peripheral apparatus and acorresponding jack socket connector of a host device. However otherarrangements of connector are possible as will be understood by oneskilled in the art and the problems described above may apply to anytype of connector where at least some contacts of the peripheralconnector and device connector that do not electrically connect innormal operation may make at least a transient connection duringseparation of the connectors. The terms ‘jack plug’, ‘jack socket’,‘plug’, and ‘socket’ should therefore not be seen as limiting the scopeof the present invention and the terms have been used as an example of adevice and peripheral connectors throughout this entire document. Theembodiments will be described principally in relation to a plug andsocket arrangement having four contacts (e.g. for left and right audio,microphone and ground) but in some arrangements a jack plug or socketmay comprise more than four, for example nine, contacts. In someembodiments with mono audio there may be only three contacts. Some ringson the plug may comprise separate regions, for example two semi-circularparts. A plug may comprise multiple fingers and a socket multipleapertures. The components referred to as a plug or a socket may each beeither a male or female connector.

FIG. 3 illustrates a circuit 300 for biasing a microphone of someremovable apparatus according to an embodiment of the present invention.Similar components to those of FIGS. 1 a, 1 b, and 2 are identifiedusing corresponding reference numerals in all subsequent Figures.

The example shown in FIG. 3 shows microphone biasing circuitry 102,which may for example comprise a suitable buffer amplifier powered froma power supply and providing a voltage output based on some referencevoltage. The microphone biasing circuitry 102 may comprise othercomponents as would be well known by one skilled in the art, forproviding a desired bias for an external microphone. In some embodimentsit may be a suitably regulated power supply rail, provided by someexternal regulation circuitry or even a battery. The biasing circuitry102 is arranged to provide a bias voltage or current at a bias node NB,which is coupled via a biasing path comprising resistors 222 and 223 andthe capacitor node to provide bias to a microphone connector node. Thebias node is thus a node of the biasing path, upstream (in the sense ofapplying a bias) of the capacitor node, at which there is sufficientelectrical bias applied to ensure a suitable bias, in use, at amicrophone connector node. The microphone connector node is a node whichis located in the biasing path at which bias for the external microphoneis received. The microphone connector node may be a node which isconnected to the device microphone contact of the device connector, suchas a microphone socket contact 117 as described above in relation toFIGS. 1 a, 1 b and 2, or the microphone connector node may comprise thedevice microphone contact itself. When a removable peripheral apparatusis connected an external microphone may be electrically connected to themicrophone connector node, e.g. via a microphone plug contact of a jackplug of the removable apparatus.

In the example of FIG. 3 the microphone connector node (NM) can be seenas the microphone contact in the jack socket, and subsequently will bereferred to as a microphone socket contact 117. Similarly the microphonecontact on the jack plug which, in use connects to the microphone socketcontact 117, will be referred to as a microphone plug contact.

As described above in relation to FIG. 2, resistors 222 and 223 arelocated in the electrical path between the microphone biasing circuitry102 and the microphone socket contact 117, i.e. the biasing path (NB toNM), to provide appropriate biasing. Also capacitor 225 is connectedbetween the biasing path and a reference voltage node, in this example aground node. Capacitor 225 is connected to a capacitor node (NC) 304 inthe biasing path between the two resistors 222 and 223 to implement alow pass filter as described above to remove any background noise, e.g.resulting from telecommunication circuitry. As mentioned above capacitor225 may be a relatively large capacitor in order to implement a suitablefilter, for example of the order of several tens of microfarads.

The embodiment shown in FIG. 3 also comprises first switch 301 which isplaced in the biasing path, between the capacitor node 304 and themicrophone socket contact 117, i.e. between nodes NC and NM. In thisparticular embodiment, the switch is placed in the biasing path betweenresistor 223 and the input signal path, i.e. the electrical path, viacapacitor 124, by which the input circuitry 103 receives signals fromthe external microphone from the microphone socket contact.

In use when a jack plug is removed from the socket, which containsmicrophone socket contact 117, switch 301 is turned off, i.e. opened,thereby disconnecting the capacitor node 304 and thus capacitor 225 fromthe microphone socket contact 117. In other words the biasing path tothe microphone socket contact is disabled i.e. the path from NC to NM isopen circuit. Therefore the charge on capacitor 225 is prevented fromflowing to the microphone socket contact 117 and creating an unwantedbias and thus audio artefact on removal of the headset.

The switch is controlled by control circuitry, not shown in FIG. 3,which operates in response to detection circuitry providing anindication of the onset of removal of the jack plug from the socket,i.e. that the jack is no longer fully inserted. The detection circuitrymay include a mechanically operated switch as known and as describedabove. Typically such known jack detect arrangements operate (toindicate that a jack plug has been inserted) only when the jack plug isfully inserted into the socket. For instance the mechanical switch mayoperate only when the tip of the plug is correctly in position. Such ajack detect arrangement will therefore provide an indication ofdisconnection as soon as the jack plug is partially removed from thesocket. Detecting disconnection of the device and peripheral connectorstherefore refers to detecting the onset of separation of the connectors(rather than detecting that the jack plug has been completely removedfrom the socket). This jack detect signal may be generated as soon asthe tip portion of the plug moves from position—which may be before atleast some contacts on the plug have been completely disconnected fromthe relevant contact of the socket.

The skilled person will, of course, appreciate that there are variousways that one could additionally or alternatively detect the onset ofremoval of the plug from the socket and that the microphone plug contacthas been, or is in the process of being, disconnected from themicrophone socket contact. Such detection could be optical, mechanical,electrical (inductive, resistive, capacitive) or even magnetic forexample. For example microphone detection circuitry may be arranged tomonitor the microphone connector node to detect, from the electricalproperties of said node, for instance the presented impedance to ground,whether or not the microphone plug contact is electrically connected tothe microphone socket contact and thus whether the jack has beenpartially de-inserted. In other words the electrical disconnection ofthe microphone socket contact from the microphone plug contact may bedirectly detected. The electrical properties of other device connectorcontacts could additionally or alternatively be monitored to detectdisconnection.

The first switch is thus operated, following such detection, such thatthe biasing path is disabled before another plug contact, e.g. one ofthe speaker contacts on the plug, makes a connection with the microphonesocket contact. In other words the bias is removed from the microphonesocket contact before the next contact on the plug comes into contactwith it. Tests have shown that a period of about 100 μs betweendetection of disconnection and the first switch being operated issufficient in most cases. Thus in some embodiments the circuit may beconfigured so that the time between detecting disconnection of theexternal microphone and operation of the first switch is of the order of100 μs or less.

The switch 301 can be thought of as isolating the microphone socketcontact 117 from capacitor 225 acting as a source of bias, and therebynot allowing the capacitor to discharge via the microphone socketcontact 117 and create an unwanted bias.

The first switch 301 may be any form of switch which is able to disablethe biasing path when required without significant impact on themicrophone bias when the biasing path is enabled (although clearly thebias supplied to the bias node NB may be supplied taking the presence ofthe first switch into account to ensure the correct bias at themicrophone connector node). In some embodiments the switch could beimplemented by more than one switch element. A transmission gate, i.e. apair of complementary MOS transistors, is one particularly useful switchimplementation.

Typically the biasing circuitry 102 and input circuitry 103 areimplemented as integrated circuits, and often as part of the sameintegrated circuit (IC). The capacitor 225 and resistors 222 and 223 areoften external to the integrated circuit(s) and coupled thereto. Theswitch 301 may conveniently be implemented as part of an integratedcircuit which includes the biasing circuitry 102 and/or input circuitry103, but in some embodiments the switch 301 could be implemented as anexternal component connected in a suitable biasing path between anintegrated circuit comprising the biasing circuitry 102 and themicrophone socket contact 117.

The circuit 300 also has input circuitry 103, which via an input pathincluding DC blocking capacitor 124, senses the a.c. signal produced bythe external microphone at the microphone connector node. It can be seenthat in this embodiment the first switch does not form part of the inputpath and thus might not be expected to have any effect on the a.c. inputsignal. However the output impedance of the microphone may be of theorder of 100Ω, and the value of resistor 223 may be of the order of 1kΩ, and the on-resistance of switch 301 may be of the order of 1000. Tofirst order, the microphone output impedance and the extra switchresistance would only give a constant gain error. However, the switch301 may for example comprise MOS transistors, with gates driven fromsignal-independent voltages, for example a supply (1.8 v), i.e. logic 1,and a ground (0 v), i.e. logic 0, while the source and drain voltagesmay be modulated by the a.c. signal. Thus their gate-source voltages,and hence their on-resistances, will vary with the signal modulation.The consequent variation of the gain modulation will thus introducedistortion. Also these non-linear impedance effects may render the inputsensitive to modulated RF signals, which may be demodulated by thesenon-linearities and produce audio-band interference. Also the switch 301is directly connected to the microphone socket contact, and may thussuffer ESD events, and thus require either extra area to make adequatelyrugged or even extra external ESD protection components.

FIG. 4 illustrates an alternative position for a switch 301, which wouldstill operate in the same fashion and disconnect the capacitor node 304from the microphone socket contact 117, in response to a jack plug beingremoved from the socket.

FIG. 4 shows that the switch 301 may be placed between the resistor 223and the capacitor node CN 304. In this position, when the switch 301 isturned off, i.e. is opened, the capacitor 225 will be isolated, i.e.disconnected, from the microphone socket contact 117. This arrangementmay be preferred to that shown in FIG. 3 because the switch poles, i.e.the source and drain of constituent MOS switches, will now be held at asubstantially constant voltage due to the filtering effect of capacitor225, and thus show much better linearity. Thus any signal distortion orRF demodulation problems are greatly mitigated. Also there is now aresistive impedance of 1 kΩ or so between the ESD-prone socket contactand the MOS switches which may provide adequate protection without thecost of extra precautions. Thus locating the switch between resistors222 and 223 may mitigate or avoid the aforementioned possible problems.

As an alternative switch 301 could be positioned so that it forms partof the biasing path and also part of the input path, as will bedescribed in more detail later in relation to FIG. 7.

FIG. 5 a shows a further embodiment in which in addition to the firstswitch 301 there is second switch 501 for connecting the microphoneconnector node (NM), i.e. the microphone socket contact 117, to a groundnode. The second switch 501 is controlled by the control circuitry andis turned off, i.e. open, in normal operation when a microphone iselectrically connected to the microphone socket contact. The secondswitch 501 is turned on, i.e. closed, when the onset of removal of thejack plug from the socket is detected. The second switch 501, whenclosed, clamps the microphone socket contact 117 to a ground node andthereby reduces the unwanted bias at that point.

The second switch 501 prevents the voltage of the microphone socketcontact 117 from floating, and by providing a defined reference voltage(i.e. ground) will ultimately stabilise the bias of the microphonesocket contact 117 at the defined reference voltage. Given the firstswitch 301 disconnects the biasing circuitry 102 and the relativelylarge capacitor 225 from the microphone socket contact 117, the voltageat this node can be rapidly driven to ground.

Thus the second switch 501 may prevent any leakage currents appearing onthe microphone socket contact and passing though say a speaker duringjack disconnect, and may also discharge any parasitic capacitancecoupled to the microphone socket contact through itself rather thanthrough a speaker. More importantly it may also suitably discharge theDC blocking capacitor 124 in implementations such as shown in FIGS. 3and 4 in which switch 301 is in the bias path between the capacitor node304 and the blocking capacitor 124, i.e. where operation of switch 301does not disconnect the d.c. block capacitor 124 from the microphonesocket contact.

FIG. 5 a also shows an optional third switch 502 which is connected inparallel with the capacitor 225, i.e. between the capacitor node 304 andthe reference voltage (ground in this instance). This third switch 502can be used to discharge the capacitor 225, in response to the detectionof disconnection of the jack plug. The switch 502 is turned on, i.e.closed, to provide a route to ground for the charge on the capacitor225. To fully discharge the capacitor 225 within say 100 μs wouldrequire a switch of low enough resistance and robust enough to carry apeak current of the order of one ampere, and capable of absorbing theenergy stored in the capacitor and a low enough on-resistance. This isuneconomic for most applications due to the silicon area required for alow Rds-on MOS switch. However, in conjunction with the switches asdescribed, it may be desirable to allow a controlled discharge of thecapacitor, say limited by a resistance in series with any switch, forexample on system power-down.

It will therefore be clear that the second and third switches may bothbe present in addition to the first switch but in some embodiments thesecond switch may be present but not the third switch or the thirdswitch may be present without the second switch. Note that if thebiasing circuitry 102 was disabled and the second and third switchescould together quickly discharge the capacitor 225 and clamp themicrophone connector node to ground the first switch may not berequired. However as mentioned this would practically require a veryrobust and low resistance second switch and so the presence of the firstswitch to disable the biasing path is preferred in most applications.

FIG. 5 b illustrates a common microphone input amplifier configuration.Op amp OA1, feedback resistor R_(f) and input resistor R_(i) constitutea standard op amp inverting amplifier configuration with gain−R_(f)/R_(i). A suitable reference voltage V_(BR) (typically chosen tobe half the supply voltage in order to maximise the output signal swing)is applied to the non-inverting input (+) of the op amp, establishingthe inverting input (−) at the same voltage and, since there is no d.c.current through R_(i), also establishing the right-hand side of theblocking capacitor C_(B) 124 at voltage V_(BR). The other plate ofcapacitor C_(B) 124 will be biased by the quiescent output voltage ofthe microphone to a voltage V_(M), which may be smaller or greater thanV_(BR) depending on the characteristics of the microphone and the choiceof V_(BR). For thermal noise reasons, inter alia, resistor R_(i) willtypically be of the order of 1 kΩ to 10 kΩ, so in order to passlow-frequency audio signal components, capacitor C_(B) may be of theorder of 1 μF. Also shown are diodes D_(P) and D_(N) to supply andground respectively, which are required for ESD and possibly latch-upprotection.

While the jack is being disconnected, even after switch 301 isolates thebias source and capacitor 225 from the microphone socket contact,capacitor C_(B) remains connected to the microphone socket contact. Theright hand side of C_(B) will still be pulled to V_(BR) via R, by the opamp feedback, so the left hand side will initially be at V_(M). In theabsence of second switch 501, this node may be discharged to ground bypaths through the speakers as described above as the jack contacts slidepast the socket contacts. If switch 501 is present and is activatedduring jack de-insert, switch 501 will take most of the dischargecurrent and mitigate this effect.

However, even with switch 501, there are still two issues. Firstly,since the op amp end of R_(i) is held at V_(BR), say 1V, and the switch501 will have a non-zero on resistance R₅₀₁, say 10Ω versus a value ofsay 1 kΩ for R_(i), then there will still be a voltage of aboutV_(BR)*R₅₀₁/(R₅₀₁+R_(i))=10 mV appearing across switch 501 and hencebetween the microphone socket contact and ground.

Also, some microphones are designed to operate with a bias voltage ofsay 2.5V. With V_(BR)=1V this implies a voltage of 1.5V across C_(B). Ifthe left hand side of C_(B) is suddenly pulled down to 0V by switch 501,the right hand side will attempt to jump down to −1.5V. However the ESDdiode D_(N) will then forward bias, and will actually clamp theright-hand side of C_(B) to about −0.6V. Thus the current will belimited only by the resistance of switch 501 and the low resistance ofthe ESD diode, giving a much larger voltage spike on the microphonesocket contact until switch 501 discharges C_(B) to at least the levelnecessary to substantially turn off the ESD diode.

FIG. 5 c illustrates two circuit improvements to mitigate these effects.

Switch SW1 is added into the path between the blocking capacitor C_(B)124 and the source of bias presented by the amplifier, in this case theinverting op amp input (−) or virtual earth. If this switch SW1 isopened during jack withdrawal, and switch 501 is activated, then switch501 no longer discharges C_(B) in series with R. Neglecting anyparasitic capacitances or leakage paths, it will essentially drive intoan open circuit and will pull down the left hand plate of C_(B)immediately.

Note SW1 is shown on the right hand side of R. If on the left hand sideas illustrated in FIG. 5 d, it would still isolate C_(B) from the sourceof bias. However for reasons of ESD ruggedness, distortion, and RFIdemodulation it is generally preferable to place SW1 on the other sideof R_(i) than the IC terminal pad and on a node whose voltage does notvary with signal. However if there are multiple microphones or inputs,and it is desired to use the same R_(i) for each to save component countor chip area, then the pad side of the resistor may be preferable asshown in FIG. 5 d as there may be additional switches (e.g. SW2 fordifferent input paths).

Depending on the architecture of the input amplifier, any switchisolating the non-microphone plate of blocking capacitor C_(B) 124 froma source of bias may be in a different position in the circuit inquestion.

Also the single ESD diode D_(N) to ground may be replaced by a pluralityof diodes (D₁-D₃) in series as shown in FIGS. 5 c and 5 d. If, forexample, there are three diodes in series, then no appreciable currentwould flow until at least 3×0.6V=1.8V, so there would be negligiblecurrent in the above example of a negative 1.5V kick on this node.Depending on the microphone and circuit requirements and specificationsand the diode characteristics, there may be only two or possibly morethan three diodes in the low-side ESD stack.

To save power, the input amplifier may be powered-down when there is nomicrophone attached. This may be by means of a control signal applied toa control input of the amplifier as illustrated in FIG. 5 c or FIG. 5 d.

FIG. 6 a illustrates an embodiment of the present invention whereby tofurther reduce the unwanted power consumption in the circuit themicrophone biasing circuitry 102 is disabled, e.g. becomes a highimpedance output, by the control circuitry 601 in response to thedetection circuitry 602. The detection circuitry detects disconnectionof device and peripheral connectors, i.e. in effect, when the jack plugis being removed from the socket and hence when switches 301 and 501should be operated. There are various ways in which the detectioncircuitry may operate as briefly disclosed above. For instance thedetection circuitry 602 may monitor a signal from the jack plug detectcontact 111 and generate a detection signal when the plug is moved, atleast partly from the fully inserted position, i.e. mechanically parted.Alternatively the detection circuitry 602 may monitor the electricalproperties of the microphone socket contact to determine when aconnection with the microphone plug contact is broken and then generatea detection signal. The detection signal could be in the form of a logicsignal operated and/or generated by a mechanical switch. This detectionsignal is received by the control circuitry which generates suitablecontrol signals for disabling the microphone biasing circuitry 102 andinput circuitry 103, as well as opening switches 301 and closing switch501.

This control signal may be generated by a command on the user interfaceand/or by action by some master controller or applications processor inthe device hosting this circuitry. Preferably the control signal may bederived from or be identical to the control signal which controlsswitches 301 and/or 510. These control signals may be generated bystraightforward logic or buffering from the jack-detect signal, or maybe modified by software-driven or hard-wired processing in the sameintegrated circuit as the input amplifier and/or microphone bias, or inanother integrated circuit in the host device, for example anapplications processor or similar.

Microphone biasing circuitry 102, input circuitry 103 and switch 301 areall thus enabled in normal use with a microphone connected and aredisabled upon detection of disconnection of the device and peripheralconnectors. The same control signal, which may for example be a 1-bitcontrol signal, may therefore be supplied to all these components.Switch 501 has the opposite operation and thus is only enabled when theonset of removal of the jack plug is detected. Inverter 603 may beincluded to invert the 1-bit digital signal for switch 501.

The discussion above of the various embodiments has assumed forsimplicity that the various components connected to ground nodes willall be the same voltage. However in practice the socket, i.e. the deviceconnector, may be some distance away from at least some of the othercircuitry, e.g. the biasing amplifier 102. Thus there may be anon-negligible impedance between a ground node near the socket and aground node near or inside an integrated circuit comprising some of thecircuitry. Any ground return current flowing across the board, forexample as a result of the pulsed supply current for TDMA RF circuitry,or return current from the speakers, may introduce a d.c. offset anda.c. ripple in the voltage at one ground relative to another.

FIG. 6 b illustrates various aspects of the invention intended tomitigate the consequences of such effects.

Consider ground node 905 to be the true ground for the system. This trueground node 905 is connected via an IC terminal pad to the on-chipground GNDC (assuming for now there is little resistance or current inthis connection). Ground node 905 is also connected via some parasiticground plane resistance R_(p) to a ground node GNDS close to the socket.Any ground return current, for example the current I_(GSM) illustrated,perhaps the 217 Hz pulsed ground return current from a TDMA RFamplifier, flowing to the ground node 905 will induce a voltage dropacross R_(p), and hence on node GNDS.

To avoid voltage ripple on the socket ground node GNDS from corruptingthe microphone output signal, the microphone input amplifier 904 may bemodified to be differential or pseudo differential as illustrated. Thevoltage at the socket ground node GNDS may also be brought on-chipwithout passing through a blocking capacitor to allow microphoneimpedance measurement functions 903.

Also this on-chip connection to the socket ground node GNDS may be usedto correct any voltage appearing at this node which is due to the returnthrough this ground node of current driven through the speakers byspeaker drive amplifier 901 (the speaker contacts are omitted in FIG. 6b for clarity).

This arrangement has two particular advantages. First, when closedswitch 501 connects to the socket ground node GNDS rather than local onchip ground node GNDC, to avoid a voltage difference between the clampedmicrophone socket contact and the locally grounded ground socketcontact. Second, as illustrated, the microphone bias may be generated bybias generation circuitry 902 with respect to the socket ground nodeGNDS, rather than the on chip ground node GNDC. (In this example, a biasreference voltage Vref, defined with respect to the on-chip ground GNDC,generates a current through resistor R1 equal to Vref/R1, which is thenmirrored and passed though resistor R2 to generate a voltage equal toR2.(Vref/R1), which is then buffered onto node NB). This avoidscapacitor 225 having to fight against a GNDC-referenced microphone biasvia resistor 222 in normal operation. A reduced size of capacitor 225may thus be possible.

Note as illustrated, switch 301 may be off-chip and the control signalBDIS for switch 301 may be generated on chip and transmitted via asuitable contact, or generated elsewhere, e.g. by an apps processor ofthe host device based on jack detect signal as described above or asgenerated elsewhere.

FIG. 7 shows a further embodiment of the present invention wherein thecircuit is operable so that either of two socket contacts (i.e. any oftwo device connector contacts) can be selectively biased by the biasingcircuitry, with the other socket contact being connected to ground. Thiscan allow the biasing circuitry to be used with removable peripheralapparatus having different arrangements of the plug contact used formicrophone (M) and ground (G).

In the example illustrated in FIG. 7 the circuit could operate correctlywith a first jack plug arrangement, having jack plug R2 contact 116 forthe microphone (M) and jack plug S contact 118 for ground (G), or asecond jack plug arrangement, having a jack plug R2 contact 116 forground (G) and jack plug S contact 118 for the microphone (M).

Microphone detect circuitry (not shown) can determine, on insertion of ajack plug, which of jack plug contacts 116 and 118 is connected to themicrophone. The control circuitry will then control switches 701, 702,703 and 704 to connect the correct jack socket contacts to the biasingcircuitry and to ground accordingly. If the external microphone isconnected to plug R2 contact 116 then switch 702 will turn on, i.e.close, and jack R2 contact 116 will be connected via socket R2 contact117 to the microphone biasing circuitry. This would mean that the groundcontact (G) of the jack plug is the S (sleeve) contact 118 and switch703 would be turned on, i.e. closed, in order to connect the S contact118 to ground. Both switches 701 and 704 would be held off, i.e. open.

If the external microphone was connected to jack S contact 118 theswitch 701 would turn on, i.e. close, and connect the S contact 118 tothe microphone biasing circuitry and switch 704 would turn on, i.e.close and connect R2 contact 116 to ground. Both switches 702 and 703would be held off, i.e. open.

On insertion of a jack plug the detection circuitry thus determines theconfiguration of the microphone and ground plug contacts and configuresthe biasing circuit 700 accordingly. Detection circuitry then continuesto monitor whether the jack plug remains inserted as described aboveand, on detection of the onset of removal of the jack plug the relevantone of either switches 701 and 702 (i.e. whichever was closed) is openedto disable to biasing path. At the same time the relevant one ofswitches 703 and 704 (i.e. whichever was open) may be closed to connectthe relevant socket contact to ground. In effect, on detection ofremoval of the jack plug, both switches 701 and 702 will be open andboth switches 703 and 704 will be closed.

In this embodiment the switches 701 and 702 are thus provided not onlyto enable a desired biasing path when required, but also to disable saidpath on detection that the jack plug is not fully inserted in thesocket.

It will be noted that in the embodiment shown in FIG. 7 the same biasingcircuitry 102, via the RC network formed by capacitor 225 and resistors222 and 223, is used for biasing either socket contact 117 or 119.

It will be noted that in this configuration, the switch used to disablethe biasing path (whichever biasing path is established) is locatedbetween the microphone socket contact 117 and a branch/tap point for theinput circuitry. Thus operation of the switch will not only disable thebiasing path but also isolate the microphone socket contact from the DCblocking capacitor 124 and thus input circuitry 103. This does howevermean that the switch forms part of the input path in normal operation,which may not be desirable in some instances. This arrangement maysuffer from RF distortion, and ESD sensitivity issues discussed above,however it may still provide the most appropriate solution in someapplications, as in the embodiment shown in FIG. 7 where it is desiredto accommodate multiple connector options.

As mentioned above, the biasing circuitry 102 and input circuitry 103may typically be implemented as an integrated circuit with capacitor 225and resistors 222 and 223 as external components. As also mentioned itmay be beneficial to implement switch 301 as part of the integratedcircuit. FIG. 8 illustrates one embodiment of such a configuration. Thedotted line marks the divide between the components which are containedwithin the integrated circuit (‘on chip’) and those which are notlocated on the integrated circuit (‘off chip’). In FIG. 8 the ‘on chip’components are to the right of the dotted line and to the left are the‘off chip’ components.

The microphone biasing circuitry 102 and the input circuitry 103 arelocated ‘on chip’, these are connected to terminals 801 and 802,respectively. Detection and control circuitry 803 which controls theenable/disable function of the microphone biasing circuitry 102 and theinput circuitry 103 is also located ‘on chip’.

The first switch 301 is, as mentioned, also located on chip betweenterminals 804 and 805. The first switch 301 in this example is shown asa transmission gate, which is constructed from parallel connected NMOSand PMOS transistors, with an transmission gate inverter INV 1 invertingthe gate drive input to the gate of the NMOS. For the transmission gateswitch 301 to turn off/close in response to the jack plug being removedfrom the jack socket a control signal (from the detection/controlcircuitry 803) will be inverted by the transmission gate inverter INV 1which will in turn stop current flowing from the source to drain of theNMOS transistor. Similarly the control signal will inhibit the flow ofcurrent through the PMOS transistor and as a result the transmissiongate 301 will prevent the flow of current from the terminal 804 toterminal 805.

The embodiment shown in FIG. 8 has the first switch 301 located in thebiasing path between resistors 222 and 223 (as illustrated in FIG. 4).Resistor 222 is coupled between terminals 801 (the output for thebiasing circuitry 103) and terminal 804. The capacitor 225 is alsocoupled into the biasing path between these two terminals. The biasingsignal output from terminal 801 is thus received back on-chip atterminal 804. As mentioned the first switch 301 is located betweenterminals 804 and 805 and thus, unless the biasing path is disabled bythe first switch 301, the biasing signal received at terminal 804 isthus output at terminal 805. Terminal 805 connects to the microphonesocket contact 117 via resistor 223.

Microphone socket contact 117 is also connected to terminal 806, whichis connected on chip via second switch 501 to a ground terminal 807.

In this embodiment of the present invention second switch 501 isrepresented by an NMOS transistor 501 which is controlled directly by acontrol signal from the detection/control circuit 803. When a jack plugis removed from the jack socket the NMOS transistor 501 will permitcurrent to flow from its drain terminal (806) to its source terminal(807). Therefore if there is any residual current, which could create anunwanted bias, remaining at the microphone socket contact and/or in thesurrounding circuitry then it will be directed to ground via the NMOStransistor 501.

FIG. 9 illustrates a host device according to an embodiment of thepresent invention showing components of the host device, including theaudio processing circuitry as well as the other processing circuitrywithin the host device. In this embodiment the biasing circuitry 102 andfirst switch 301 form part of an audio codec (although otherarrangements are possible). A removable peripheral device, in thisexample headphones (with microphone) 910, connects to the host devicevia the jack plug 10 and jack socket which, in use, is electricallyconnected to the biasing circuitry 102. Two nodes, capacitor node (NC)and the microphone connector node (NM), can be seen in the biasing pathbetween the jack plug 10 and the biasing circuitry 102. The capacitor225, which in an embodiment of the present invention, is coupled betweenthe biasing path at capacitor node NC and a reference voltage (which inthis figure is shown as ground). This capacitor 225 has a capacitance inthe microfarad range, which is used to suppress any noise arising alongthe biasing path and any other electrical connections which areconnected to the biasing path. The most common source of noise, in acommunications device, along the biasing path will arise from the RFtransmitter.

Embodiments of the invention may be arranged as part of an audio and/orsignal processing circuit, for instance an audio circuit which may beprovided in a host device. A circuit according to an embodiment of thepresent invention may be implemented as an integrated circuit and may beimplemented in a host device. The term host device is used in thisspecification to refer to any electronic or electrical device which isremovably connectable to an external peripheral apparatus having amicrophone where the biasing for such external microphone is generatedin the host device. The host device may especially be a portable and/orbattery powered host device such as a mobile telephone, an audio player,a video player, a PDA, a mobile computing platform such as a laptopcomputer or tablet and/or a games device for example. A removableperipheral apparatus is any apparatus which may be connected to and usedwith a host device and, as used in this application, refers to apparatushaving a microphone where a bias for the microphone is supplied by theexternal device when connected. The peripheral apparatus may forinstance be a headset, e.g. a set of headphones, earbuds or the likeincluding a microphone.

It will be appreciated that a jack plug is one example of a removableperipheral device which could suffer from the cross connection ofnon-corresponding contacts. Any removable peripheral apparatus where theplug contacts are arranged along the direction of insertion and removalhas the possibility of being effected by unwanted biases at certainsocket contacts and therefore the circuit described above will be usefulto mitigate any undesired effects. Furthermore any removable peripheralapparatus where the plug contacts may make contacts with any othersocket contact, whether they are arranged in a linear fashion or not,may come under the scope of the present invention. Similarly for thejack socket, any other removable peripheral apparatus socket can be usedin conjunction with the present invention.

The term node may be taken to mean any point along an electrical pathand does not necessarily imply a terminal, connection point or contactof a connector (although clearly a terminal, connection point or contactmay be a node).

The embodiments above have been described in relation to the removal ofunwanted bias from microphone circuits. However some other analogue anddigital circuits may require the removal of unwanted bias from a contactof a connector to prevent undesired effects on removal of a removableperipheral apparatus, i.e. there may be other applications where a biason a contact of a device connector making contact with other contacts ofa peripheral connector may be undesirable. The principles of the presentinvention may be readily applied to such other applications.

The skilled person will recognise that at least some aspects of theabove-described apparatus and methods may be embodied as processorcontrol code, for example on a non-transitory storage or carrier mediumsuch as a disk, CD- or DVD-ROM, programmed memory such as read onlymemory (Firmware), or on a data carrier such as an optical or electricalsignal carrier. For some applications embodiments of the invention willbe implemented on a DSP (Digital Signal Processor), ASIC (ApplicationSpecific Integrated Circuit) or FPGA (Field Programmable Gate Array).Thus the code may comprise conventional programme code or microcode or,for example code for setting up or controlling an ASIC or FPGA. The codemay also comprise code for dynamically configuring re-configurableapparatus such as re-programmable logic gate arrays. Similarly the codemay comprise code for a hardware description language such as Verilog™or VHDL (Very high speed integrated circuit Hardware DescriptionLanguage). As the skilled person will appreciate, the code may bedistributed between a plurality of coupled components in communicationwith one another. Where appropriate, the embodiments may also beimplemented using code running on a field-(re)programmable analoguearray or similar device in order to configure analogue hardware.

It should be noted that the above-mentioned embodiments illustraterather than limit the invention, and that those skilled in the art willbe able to design many alternative embodiments without departing fromthe scope of the appended claims. The word “comprising” does not excludethe presence of elements or steps other than those listed in a claim,“a” or “an” does not exclude a plurality, and a single feature or otherunit may fulfil the functions of several units recited in the claims.Any reference numerals or labels in the claims shall not be construed soas to limit their scope.

What is claimed is:
 1. A host device for use with a removable peripheralapparatus having a microphone, the host device comprising: a deviceconnector capable of forming a mating connection with a respectiveperipheral connector of the removable peripheral device, the deviceconnector having a device microphone contact; a biasing path forsupplying an electrical bias to said device microphone contact; acapacitor connected between a reference voltage node and a capacitornode of said biasing path; a first switch located in the biasing pathbetween the capacitor node and the device microphone contact, saidswitch being operable to disable the biasing path; detection circuitryfor detecting disconnection of the peripheral connector and deviceconnector; and control circuitry for controlling said first switch todisable the biasing path in response to detection of disconnection ofthe peripheral connector and device connector.
 2. A host device asclaimed in claim 1 wherein said control circuitry is configured tooperate said first switch to disable the biasing path within 100 μs ofsaid detection of disconnection.
 3. A host device as claimed in claim 1wherein said first switch is closed only when a microphone contact ofthe peripheral connector is detected to be connected to said devicemicrophone contact.
 4. A host device as claimed in claim 1, furthercomprising a second switch operable to connect said device microphonecontact to a second reference voltage.
 5. A host device as claimed inclaim 1, wherein, in use, the voltage at the reference voltage node is aground voltage.
 6. A host device as claimed in claim 5 wherein saidsecond switch is controlled by said control circuitry, and operates toconnect said device microphone contact to ground in response to saiddetection of disconnection.
 7. A host device as claimed in claim 1further comprising biasing circuitry operable to supply said electricalbias to the biasing path via a bias node wherein said control circuitryis configured to disable said biasing circuitry in response to saiddetection of disconnection.
 8. A host device as claimed in claim 1further comprising input circuitry for receiving an input signal fromsaid device microphone contact.
 9. A host device as claimed in claim 8wherein said control circuitry is configured to disable said inputcircuitry in response to said detection of disconnection.
 10. A hostdevice as claimed in claim 8 wherein a DC blocking capacitor is situatedbetween the device microphone contact and said input circuitry.
 11. Ahost device as claimed in claim 10 further comprising an input switchlocated between the DC blocking capacitor and the input circuitry, whichoperates in response to said detection of disconnection.
 12. A hostdevice as claimed in claim 11 wherein said input switch is locatedbetween a resistor and the input circuitry.
 13. A host device as claimedin claim 10 further comprising electrostatic discharge protectioncircuitry connected to an ESD node located between the DC blockingcapacitor and the input circuitry, said electrostatic dischargeprotection circuitry comprising a plurality of diodes connected inseries.
 14. A host device as claimed in claim 1 wherein said one or moreresistors comprise two or more resistors connected in series in saidbiasing path.
 15. A host device as claimed in claim 14 wherein saidcapacitor node is located between two of said two or more resistorsconnected in series.
 16. A host device as claimed in claim 14 whereinsaid first switch is located between two of said two or more resistorsconnected in series.
 17. A host device as claimed in claim 1 comprisinga third switch in parallel with said capacitor for connecting saidcapacitor node to said reference voltage node.
 18. A host device asclaimed in claim 1 comprising biasing circuitry operable to supply saidelectrical bias to the biasing path via a bias node wherein said biasingcircuitry generates said bias at the bias node with respect to thevoltage at a ground contact of said device connector.
 19. A host deviceas claimed in claim 18 wherein said reference voltage node is connectedto said ground contact of said device connector.
 20. A host device asclaimed in claim 1 further comprising: an alternative bias path forsupplying an electrical bias to an alternative device contact of saiddevice connector; and a microphone detect circuit for detecting, onconnection of the device connector and the peripheral connector whetheran external microphone is connected to said device microphone contact orsaid alternative device contact; wherein the host device is operable sothat, in use, said biasing path or said alternative bias path can beselectively enabled.
 21. A host device as claimed in claim 20 whereinsaid biasing path comprises a first path section and a second pathsection and wherein said first path section is shared with thealternative bias path and the second path section is not shared with thealternative bias path wherein said first switch is located in the secondpath section and is operated to disable the biasing path when thealternative bias path is selectively enabled.
 22. A host device asclaimed in claim 1, wherein the device connector is a socket forreceiving a jack plug peripheral connector.
 23. A host device as claimedin claim 1, wherein the host device is at least one of: a portablecomputing device; a laptop computer; a personal data assistant; apersonal media player; an mp3 player; a portable television; a mobilecommunications device; a mobile telephone; a navigation aid; a GPSdevice; a game console; a tablet computer.
 24. An integrated circuitcomprising: microphone biasing circuitry for outputting a biasing signalat a first terminal; a second terminal, for, in use, receiving saidbiasing signal via an external RC network, a third terminal, for, inuse, outputting said biasing signal to a external microphone contact, aswitch operable to connect said second terminal to said third terminal,switch control circuitry for operating said switch to disconnect saidsecond and third terminals in response to a detection that a microphonecontact is electrically disconnected from said third terminal.
 25. Abias reducing circuit, comprising, a connector for connecting anexternal microphone, said connector having a device microphone contact;and switching circuitry for isolating the device microphone contact fromany source of bias, wherein said switching circuitry operates inresponse to said external microphone being electrically disconnectedfrom said microphone contact.